The underlying mechanism(s) of opiate tolerance remains to be elucidated. Although several theories have been postulated for such phenomenon, recent findings of endogenous anti-opiate peptides (AOPs), like neuropeptide FF (NPFF), suggest the involve of an endogenous model to explain the development of opioid tolerance. NPFF is a mammalian octapeptide, that has been shown to attenuate various opiate- induced effects, like anti-nociception and the development of morphine tolerance and dependence. It is proposed that the administration of morphine releases anti-opiates as part of a homeostatic mechanism. As greater quantities of morphine are administered, increasing quantities of anti-opiates are released into the CSF. As a direct consequences mu opioid receptor (MOR) complex is down-regulated, and larger doses of morphine are required to overcome the antagonism by endogenous anti- opiates, producing progressively higher degrees of agonist sub-sensitivity (tolerance). The present project will examine the mechanism(s) by which NPFF attenuate morphine-induced tolerance. Rats will be made tolerant by chronic (13 days) intraventricular infusion of morphine sulfate, via the Alzet 2001 osmotic mini-pumps. Because previous findings have shown that chronic i.c.v infusion of morphine and NPFF alone were able to down-regulate MOR density, the present proposal represents an excellent model to further examine the mechanism(s) by which NPFF attenuates morphine tolerance in spinal cord and discrete brain regions (Striatum, Cerebral Cortex, and Thalamus) that are associated with opiate tolerance. To test this hypothesis, studies will be conducted to examine the chronic effects of NPFF to alter the MOR receptor density and messenger RNA level associated with morphine tolerance. Additional experiments will test the chronic effects of NPFF on morphine tolerance as measured by mu mediated responses of the MOR signaling transduction pathway (cAMP levels and GTP-gamma-S activity). These finding are expected to delineate the mechanism(s) by which NPFF attenuates morphine tolerance by providing a clearer understanding as to where the interaction is occurring within the complex cascade of biochemical events of the MOR system.